Ceramic arctubes for high intensity discharge (HID) lamps are fabricated from polycrystalline alumina (pca) and need seals between the niobium leadwires and the arctube. A schematic diagram of a ceramic arctube for a metal halide lamp is shown in figure 1. The properties required for these seals are very demanding. The seals must be: hermetic; able to cope with many cycles of thermal expansion and contraction; sufficiently robust to survive vibrations while the lamp is in service; resistant to the dose components within the arctube at high temperatures over the rated life of the lamp. Additionally, the process for making the seals must be robust, cost effective and the cycle time has to be compatible with the required throughput. Seals for ceramic arctubes are usually formed from a frit ring composed of various metal oxides which form a number of crystalline phases in a glass matrix after processing. The frit ring is accurately located onto the components and the assembly then subjected to a well-defined and precisely controlled heating and cooling cycle. The requirements for the frit rings are specified by weight, composition, size and shape.
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